The present invention relates to the preparation of pyrroles from pyrrolidines and of pyridines from piperidines by dehydrogenation over noble metal catalysts.
It is known that pyrrolidine and piperidine can be dehydrogenated over palladium- and/or platinum-containing supported catalysts to give pyrrole and pyridine, respectively.
U.S. Pat. No. 3 522 269 describes the dehydrogenation of pyrrolidine to pyrrole over Pd catalysts at relatively high temperatures, preferably 400-450xc2x0 C.
GB-A 1 393 086 describes the dehydrogenation of piperidine to pyridine by means of catalysts comprising palladium on SiO2 as a support.
EP-A 67 360 and 155 649 describe Pd catalysts for dehydrogenating pyrrolidines, which are effective at lower temperatures of from 160 to 400xc2x0 C.
In the dehydrogenation of pyrrolidine to pyrrole, a certain amount of byproducts, such as butylamine, N-butylpyrrole and butyronitrile, is formed. Among these, butyronitrile, which is generally formed in amounts of from 0.3 to 3% by weight, is particularly troublesome. Owing to the very similar boiling points of pyrrole and butyronitrile, butyronitrile can be separated from pyrrole by distillation only with considerable effort. Columns having a large number of theoretical plates are required for this purpose, and a high reflux ratio has to be employed. The distillative separation is associated with a large loss of desired product. However, the use of the pyrroles in the preparation of pharmaceutical active compounds or in electronic components requires very high purity of the pyrroles.
It i s a n object of the present invention to provide an improved process for the preparation of pyrroles and pyridines in which byproducts are formed to a smaller extent.
We have found that this object is achieved by a process for the preparation of pyrroles of the formula (I) 
where
R1 and R2, independently of one another, are hydrogen or an aliphatic radical of 1 to 6 carbon atoms,
by dehydrogenating pyrrolidines of the formula (II) 
where R1 and R2 have the abovementioned meanings,
in the presence of a supported noble metal catalyst, wherein the dehydrogenation is carried out in the presence of from 1 to 50% by weight, based on pyrrolidine and water, of water.
It has been found that the formation of (substituted) butyronitrile is greatly suppressed by the presence of water during the dehydrogenation of (substituted) pyrrolidine. The amount of butyronitrile in the crude discharge of the dehydrogenation of pyrrolidine is in general  less than 0.2, preferably  less than 0. 1, % by weight. As a result, the subsequent purification of pyrrole by distillation is considerably simplified. Large losses of desired product do not occur, making the preparation of pyrrole as a whole more economical.
Suitable pyrrolidines (I) are unsubstituted pyrrolidine and pyrrolidine which is substituted in the 3- and/or 4-position by an aliphatic radical of 1 to 6 carbon atoms. The aliphatic radicals may be substituted by groups which are inert under the reaction conditions. Preferred aliphatic radicals are methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl and isomeric pentyl and hexyl radicals. A preferred starting compound of the novel dehydrogenation is unsubstituted pyrrolidine.
In the same way, the preparation of pyridines of the formula (III) 
can be carried out by dehydrogenating piperidines of the formula (IV) 
Therein, R3, R4 and R5, independently of one another, may have the same meanings as R1 and R2. Unsubstituted piperidine is preferred. As a result of the presence of water during the dehydrogenation, the formation of (substituted) valeronitrile as a byproduct is accordingly suppressed.
The dehydrogenation is carried out in the presence of from 1 to 50, preferably from 5 to 50, particularly preferably from 5 to 25, % by weight of water. The percentage data are based on the sum of pyrrolidine or piperidine and water, which gives 100% by weight. The water may have been admixed with the pyrrolidine or piperidine feedstock stream or may be fed into the reactor during the dehydrogenation in the corresponding ratio.
The novel catalytic dehydrogenation can be carried out in the liquid phase or in the gas phase.
The novel catalytic dehydrogenation can be carried out in the liquid phase. For this purpose, pyrrolidine or piperidine and the noble metal-containing supported catalyst, if required in a solvent, are kept at the reaction temperature, for example for from 1 to 10 hours. The catalyst is separated from the resulting product mixture by filtration, and pyrrole or pyridine is obtained by fractional distillation. Preferred solvents are toluene, xylenes, diethylbenzene, ethanol, n-butanol, methyl glycol, ethylene glycol and 1,2-dimethoxyethane. The dehydrogenation in the liquid phase is usually carried out in the presence of small amounts of hydrogen in order to activate the catalyst.
The novel dehydrogenation is preferably carried out continuously in the gas phase, it being possible to arrange the catalyst in a fixed bed or as a fluidized bed.
If the novel dehydrogenation is carried out in the gas phase, the pyrrolidine/steam mixture or piperidine/steam mixture is preferably present in a carrier gas stream comprising nitrogen and/or hydrogen. The volume ratio carrier gas: pyrrolidine or piperidine is as a rule from 1:1 to 100:1, preferably from 2:1 to 50:1, particularly preferably from 3:1 to 40:1. The volume ratio of nitrogen to hydrogen may be from 0.01:1 to 100:1, preferably from 0.1:1 to 10:1, particularly preferably from 0.5:1 to 5:1.
The novel dehydrogenation can be carried out in the presence of all noble metal-containing supported catalysts which are customary for this dehydrogenation reaction. In general, temperatures of from 150 to 400xc2x0 C. and pressures of from 0.01 to 50 bar are employed.
The novel dehydrogenation can be carried out, for example, in the presence of the palladium supported catalysts which are described in EP-A 0 067 360 and contain basic compounds and/or elements of group 11, group 12 and group 7, cobalt and/or nickel. Supports are, for example, alumina, silica, aluminum silicate, magnesium silicate or spinels of aluminum, of chromium or of iron. Preferred activating additional elements of groups 7, 11 and 12 are manganese, zinc and silver. Preferred basic compounds are the oxides, hydroxides or carbonates of the alkali metals, preferably of lithium, of the alkaline earth metals, preferably of magnesium and of calcium, and of the lanthanides, preferably of cerium, of praesodynium and of neodynium.
However, noble metal-containing catalysts which contain from 30 to 100% by weight of
a) palladium on an oxide of a rare earth metal or an oxide of an element of group 4 or
b) a platinum/palladium mixture on alumina, an oxide of a rare earth metal or an oxide of an element of group 4
and from 0 to 70% by weight of alkali metal oxide or alkaline earth metal oxide are preferred. In the presence of these catalysts, the novel dehydrogenation is generally carried out at from 150 to 300xc2x0 C., preferably from 170 to 270xc2x0 C., particularly preferably from 180 to 250xc2x0 C., and from 0.01 to 50, preferably from 0.1 to 5, particularly preferably from 1 to 1.5, bar.
In the context of this description, suitable rare earth metals are the elements of the lanthanide and the actinide group of the Periodic Table of the Elements, such as lanthanum, cerium, neodynium, samarium, gadolinium, ytterbium, actinium, thorium, uranium and neptunium, cerium, praesodynium, neodynium, samarium, europium, terbium, ytterbium, thorium and protactinium being preferred, and cerium, praesodynium, neodynium and thorium being particularly preferred.
Suitable metals of group 4 are titanium, zirconium and hafnium, titanium and zirconium being preferred, and zirconium being particularly preferred. Suitable alkali metals or alkaline earth metals are lithium, sodium, potassium, cesium, beryllium, magnesium, calcium, strontium and barium, sodium, potassium, magnesium, calcium and barium being preferred.
The active components of the catalyst (noble metals) are preferably present on oxides of the lanthanides or actinides or on oxides of elements of group 4 when the active component is pure palladium, and preferably on supports substantially comprising alumina, on oxides of the rare earth metals or on oxides of group 4 when the active component is a platinum/palladium mixture.
The catalysts can, if present, be prepared by kneading of the additives (i.e. of the alkali metal or alkaline earth metal oxides) together with the support material, thermal aftertreatment (heating) at from 400 to 900xc2x0 C. and impregnation with a solution containing a salt of the noble metal, or by impregnation of the support with a solution of the additives and of the noble metal, for example in the form of solutions of their nitrates, chlorides, formates, oxalates or ammoniates and subsequent heating at from 400 to 900xc2x0 C. If spinel formation is to be effected, a temperature of from 900 to 1300xc2x0 C. must be reached after the kneading or the impregnation of the alumina with the oxide or the solution of the additive component (cf. Ullmanns Encyklopxc3xa4die der technischen Chemie, 3rd Edition (1955), Volume 6, pages 242 to 244).
The noble metal content of the catalysts is as a rule from 0.0001 to 25, preferably from 0.001 to 20, particularly preferably from 0.05 to 15, % by weight, based on the support material. The catalysts can be used, for example, in the form of moldings, e.g. extrudates, tablets or rings, or as powder, depending on the intended use. The preferably used catalysts have the advantage that they are more slowly deactivated and that, through their use during the dehydrogenation in industrial operation, substantially fewer stoppages or downtimes are therefore required for catalyst change. They have a high initial selectivity for the dehydrogenation to give the aromatic heterocycle, so that their selectivity is also very good over the total time-on-stream with a catalyst batch. The formation of pyrrolines as byproducts can be substantially suppressed.
The reaction is preferably carried out in a tubular reactor since it is important to avoid back-mixing from the exit of the plant, i.e. the end of the last reaction zone, to the entrance, i.e. the beginning of the first reaction zone, which can most easily be achieved in tubular reactors and the plug flow forming therein.
Water is separated from the resulting product mixture by phase separation, for example in a mixer-settler apparatus. Pyrrole and pyridine, which may still contain unconverted pyrrolidine and piperidine, respectively, and byproducts, are then subjected to purification by distillation.
The compounds prepared by the novel process can be used, for example, for the preparation of pharmaceutical active compounds. The pyrroles prepared according to the invention can be used in particular in electrolytic capacitors. Pyrroles and pyridines are useful intermediates for organic synthesis.